The present invention relates to distance measuring apparatus using an acoustic signal and more particularly to apparatus provided additionally in a non-destructive examination equipment for measuring a long distance, for example, to an objective.
Usually, a large-sized pressure vessel is formed by welding thick steel plates. When the pressure vessel cannot be observed visually due to the coverage of the vessel with a thermal insulation material, a shielding wall, etc., an examination equipment is used which includes a vehicle with magnetic wheels and caterpillars of a magnetic material and a device mounted on the vehicle for examining the weld in a non-destructive manner to confirming whether the weld in the vessel is complete or not. The wall surface vehicle for examination examines the weld along the welding line.
In the non-destructive examination, it is necessary to accurately measure the position of the vehicle for an examination of a vessel examination equipment. Such techniques are disclosed, for example, in JP-A-51-95888 (corresponding to U.S. Pat. Nos. 3,988,922; and 4,010,636), JP-A-60-102580 and 60-102581 related to each other, and JP-A-1-291157.
FIG. 21 schematically illustrates the disposition of the vehicle for the examination of a vessel examination equipment (assume that the device is not of a self-propelled type for convenience of explanation in FIG. 21) for measuring the position of the device using an acoustic signal. In FIG. 21, broken lines show a weld (line) to be examined.
For example, an examining device 802 is set on a pressure vessel 1. A transmitting probe 803 as position detecting means attached to the examining device transmits an acoustic signal 805 having a frequency, for example, of 350 KHz from the vessel surface into the vessel. The acoustic signal 805 propagates through the vessel at a well-known sound velocity and is received by a receiving probe 804 fixed to the vessel at a time corresponding to the distance. Thus, the distance between the receiving probe 804 and the transmitting probe 803 (examining device) is measured.
When the distance between two points is measured using an acoustic signal, generally, the transmitting probe and the receiving probe use the same-frequency signal. As the transmission frequency increases, the attenuation coefficient increases and the acoustic signal does not arrive at a distant point. Therefore, it is technically difficult to use an acoustic signal having a high frequency to measure the position of the examining vehicle on a large-sized pressure vessel. However, conversely, the measuring accuracy on the propagation distance increases as the frequency used increases.
In the JP-A-51-95888 techniques, a low frequency (350 KHz) is used to cause the acoustic signal to arrive at a distant point. Therefore, the measurement of a propagation distance with high accuracy is difficult.